Put simply, this is a vacuum line flow blockage. What I hope to accomplish in this write up is share some of the steps I have taken to try and locate the source of the problem and eliminate the troublesome Check Engine light for emissions testing.

Symptom: When the car is fully warmed up, it idles rough like it wants to stall out and eventually does die. The idle never seems to hold, it just wants to stall out constantly.

On my car, I threw two P0400 codes. I have replaced many things in hopes of curing it, I bought a $74 EGR valve I didnít need, I bought a $137 inlet manifold sensor(MD305600) I didnít need, but none the less, I shouldnít have to worry about those items failing until another ten years. The culprit was an unusual insert by the factory. I have the OEM factory vacuum lines and unbeknownst to me was a tiny brass regulator stuffed inside the rubber hose that is connected between the vacuum control valve and the EGR solenoid. This brass insertsí outside diameter measured .146 of an inch and the tiny hole inside measured .020 on an inch! I could not stick a needle through the hole successfully. Needless to say it was blocked with carbon deposits and I replaced the entire line completely. Finding this solution did not come easy, it took many nights of reading many car forums whose owners produced the same code on their OBDII car. The next post below is what inspired new problem solving directions. Below are some of the common trouble shooting tips that not only worked for me, but seem to be the cause of the nightmarish P0400 code. This is applicable to the SOHC 3/S.

Start with the minor things first!Vacuum Lines
Check each and every vacuum line for leaks, make sure they are not cracked and have no blockages, using a vacuum test tool or simply blow through them to verify. I bought a new 3 foot vacuum line to blow through each and every line to check for blockages, thatís how I found the brass insert, and mostly my fault, it was the last procedure I performed, yet the simplest!

Clean Throttle Body
Disconnect the intake hose and using a stiff toothbrush, spray some choke/carb cleaner on the brush and scrub the upper side of the throttle body where the vacuum line holes are. Clear all the carbon deposits away. Using a clean rag, stuff it through the butterfly. Then take the choke/carb cleaner and spray it through the vacuum lines on the throttle body. The excess cleaner will drip and soak up on the rag inside the throttle body. Blow through each line after cleaning to make sure those lines are not blocked. Keep cleaning until they are clear.

A little more into itCheck Components
Check the Vacuum Control Valve to make sure there is flow. Take out the solenoids and using a 9V battery, touch to the two electrical contacts with the battery to make sure you can hear a clicking sound, that means they are working. Use a can air to blow out any loose carbon deposits inside the solenoids. Take off the EGR valve to check and see if the diaphragm is movable or ruptured. The diaphragm should move smoothly, you should be able to connect a hose and suck air to move the EGR valve. The thing about the EGR valve is that it uses a rubber diaphragm and it sits right over the exhaust manifold so its constantly pounded by heat and the rubber could crack breaking the vacuum seal. Anything not working will have to be replaced.

Inlet Manifold Sensor(a.k.a. MDP Sensor)
I was surprised to hear that the dealer stocked many of these expensive nothing-to-it of a sensor. Apparently these things die out pretty frequently. It is tied to the EGR system because it monitors when the EGR valve is doing itís job. If the EGR valve is not doing itís job, the Intake Manifold Sensors Job is to report to the ECM that there isÖ.NO FLOW! But itís a good idea to clean inside the sensor area. Take it off the intake manifold and using a can of air, clean out the loose crap that somehow gets in there. As far as testing the sensor, my book stops at 1996 and I own a 1997. I havenít a clue what the ohms test are for the three electrical contacts but I ended up replacing it. After all of this, the code went away and the car ran normal again.

Here is an aricle posted by an experienced technician on the code, this is what prompted me to look in other directions, even though it's based on the Chevy Tracker, so I thought I would share.
_______________

ď An EGR system can be monitored by several methods, including checking the temperature of the exhaust gas when the EGR valve is opened, sending a pintle position feedback voltage to the power control module (PCM), or monitoring the change in O2 sensor voltage or intake manifold pressure when the EGR valve is opened. Measuring the change in manifold pressure is the method used by this Tracker. Ē

Anyone who has worked on a Chevrolet Tracker knows that fixing an exhaust gas recirculation (EGR) code P0400 can be a real nightmare. This was the case recently when I took a call on a 1997 Tracker with a 1.6 liter engine. It was evident the technician had years of experience, good equipment and was determined to find out why this code kept returning. The technician's frustration was dwarfed by the owner's - a trail of shops had been unable to fix his problem, so the heat was on.

An EGR system can be monitored by several methods, including checking the temperature of the exhaust gas when the EGR valve is opened, sending a pintle position feedback voltage to the power control module (PCM), or monitoring the change in O2 sensor voltage or intake manifold pressure when the EGR valve is opened. Measuring the change in manifold pressure is the method used by this Tracker. The P0400 code is set when the EGR valve is commanded wide open and the expected change in manifold pressure measured by the MAP sensor does not occur.

This EGR system uses several components. The EGR vacuum solenoid is controlled by the ECM and is turned on to allow vacuum to the pressure transducer - it basically turns the system on or off. The pressure transducer regulates the amount of vacuum to the EGR valve depending on backpressure in the exhaust (engine load).

Unless at wide-open throttle, the more the load, the more the EGR valve opens. The last part of the system is the EGR bypass solenoid, which is activated by the ECM during a system test to apply full manifold vacuum to the EGR valve. When this occurs, the maximum amount of exhaust gas flows into the intake manifold, decreasing manifold vacuum. This event causes an increase in voltage output from the MAP sensor, monitored by the ECM. A code P0400 will be recorded if the voltage change is not within a specific "window."

This engine had a new EGR valve, new pressure transducer, new EGR solenoid vacuum valve and a new EGR bypass valve. All vacuum hoses were connected correctly. The MAP was working properly. The ECM was operating the solenoids as intended, but the P0400 code continued to set. It was at this point that the technician called our hot line.

After discussing the problem, it was evident the system was working correctly, so the problem had to be related to the amount of exhaust gas entering the intake manifold. This becomes more believable when looking at the route the exhaust gas takes on its way to the intake. The gas is brought into the system from the No. 4 cylinder exhaust manifold. From there, it winds through the back of the cylinder head into the intake manifold and to the EGR valve. Then it goes back into the intake manifold and up an EGR transfer tube. From the transfer tube, it takes a turn and gets vented into the intake air stream behind the throttle plate. It's amazing the system works at all!

The technician had lifted the EGR valve at idle in an earlier test. This caused the engine to almost stall so he assumed the passageways were not plugged. He was right; they were not plugged, but they were restricted, and that slight restriction caused the code to reset repeatedly. As it turned out, the EGR port behind the throttle plate was coated with carbon. This is a common problem when hot exhaust gas makes contact with cool air flowing though the intake manifold. The rapid change in temperature causes the carbon to condense at the port outlet.

How could one small restriction affect the entire system? Think in terms of partially restricting a 50-foot section of garden hose with a needle nose vise grip. Although the pinched-off section is small, the flow through the entire hose is reduced. That same principle applies to this problem. The fix was to clean out the port behind the throttle plate with a No. 3 Phillips screwdriver. This increased the flow of exhaust gas enough to allow the EGR system to pass the self-test. The troublesome P0400 never set again.

Dave Martin is an IDENTIFIX GM specialist. He is ASE master and L1 certified, with 27 years of diagnostic repair experience.

I think i have the same headache. Where is the vacuum control valve you speak of?

Its a small vacuum line that goes into a three way elbow down under the rest of the vacuum lines. I just went out and replaced mine today to hopefully fix my problem. I blew through the lines and that one had no airflow. Also you can feel the insert inside of the line if you squeeze with a pair of pliers. I just pushed a small screwdriver inside and pushed the fitting out of the line to see if it fixes the problem. I also have an 98 SL, not sure how much different on the VR4

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